A coordinated program of physiological and morphological experiments is planned to acquire greater understanding of the events involved in the development and specification of synaptic connections. Electrophysiological experiments are performed on identified developing nerve-muscle junctions and followed by the following proposed experiments: (1) to demonstrate systematically the maturational changes in the transmitter release properties that occur during the development of the amphibian nerve-muscle junction, to determine the chronology of these nerve terminals and the postsynaptic muscle fiber during ontogeny and to correlate these with the changes in the nerve terminal's physiology, and (3) to determine what factors control the specificity of formation and elimination of synapses during neuromuscular development.